Superconducting wires or tapes have been developed based upon high temperature superconducting (HTc) materials which may have critical temperatures TC above 77 K, facilitating their use in cryogenic systems cooled by liquid nitrogen. In certain applications, such as use in superconducting fault current limiters (SCFCL), high temperature superconducting (HTS) tapes may experience high temperature excursions in the case of a fault, in which the superconducting layer undergoes a transition to non-superconducting state. To accommodate faults HTS tapes include a stack of one or more metal layers that can conduct excess electrical current when a superconductor layer becomes non-superconducting.
The synthesis of HTS tapes involves many challenges including the need to form a complex stack of materials that constitute the HTS tape. Often, a superconductor layer of the superconductor tape is formed on a metallic substrate that is in the form of a ribbon or tape structure which serves as the template for growth of necessary layers for forming the superconductor tape. The metallic substrate is often processed by drawing the tape through a series of deposition and processing chambers that are used to form the multiple layers on the metallic tape. In order to provide sufficient current carrying capabilities in the resultant superconductor tape, the crystalline superconductor material is grown in a manner to promote a specific crystallographic orientation or “texture” of the resulting layer. The conventional HTS crystalline superconductor material is chosen from a class of layered complex oxides, in which current carrying copper oxide layers are oriented within a plane perpendicular to the c-axis of the crystallographic unit cell. Accordingly, it is desirable to form a c-axis texture of the superconductor tape in which the current carrying layers of the superconductor tape lie parallel to the plane of the tape. This entails the deposition of at least one intermediate layer, and often several layers, that separate the metallic tape substrate from the superconductor layer. The intermediate layers may play multiple roles including use as a diffusion barrier to prevent interdiffusion of the metallic tape material and superconductor layer, as well as use as a crystalline template from which a highly crystallographically oriented superconductor layer can be grown.
To achieve desired conduction properties, a superconductor layer thickness of two micrometers or greater may be required, which may result in excessive strain in such a layer, reducing the mechanical integrity of the superconductor layer. After formation of the superconductor layer, a metallic overlayer may be formed on the superconductor layer to serve as a conductive layer to conduct current during a fault condition in which the superconductor layer is in a non-superconducting state. Once the complete stack of layers that constitutes a superconductor tape is formed, the tape may be assembled into a current limiter by fastening tape portions together to form a set of multiple, extended, conductive paths. The tape portions are mounted in a module that provides mechanical strength and convenient handling of the superconductor tapes for assembly into a current limiter device. In view of the above it may be appreciated that the formation of superconductor tapes, in particular for current limiter applications, requires extensive and complex processing. It is with respect to these and other considerations that the present improvements are needed.